Multi-layer plastic tube

ABSTRACT

A multi-layer plastic tube comprises a base tube and a barrier layer surrounding the base tube, the barrier layer being impermeable to oxygen, and a protective layer applied on the barrier layer for protecting the barrier layer from damage, wherein the material of the barrier layer and protective layers may include a lubricant.

The invention relates to a multi-layer plastic tube which can be usedfor conducting gases and/or fluids and is particularly suited as adrinking-water, sanitary-water or heating tube.

In the field of sanitary and heating installations, use is made not onlyof metal pipes but also of plastic tubes made e.g. from polyethylene(PE). The plastic materials used in such tubes are permeable to oxygenand therefore will usually be provided with a (oxygen) diffusion barrierlayer which normally surrounds the fluid-conducting base tube on theoutside.

In practice, EVOH (ethylene-vinyl-alcohol) has proven useful as amaterial for said barrier layer. The drier this material is, the betterit is able to fulfill its function as a diffusion barrier. Unfavorably,EVOH becomes more brittle with increasing dryness. Since an increasingbrittleness is accompanied by reduced electricity, this in turn leads toa risk of fossilization and cracking and of a crumbling of the materialof the barrier layer. This danger exists particularly if the plastictube is exposed to changes in its length which are caused by variationsof the temperature of the fluid in the tube.

To protect the mostly brittle diffusion barrier layer from damage, DE296 06 533 U1 discloses a multi-layer plastic tube wherein the barrierlayer is surrounded by a protective layer. As a material of this barrierlayer, preferred use is made of PE. The material of the protective layeris softer than the material of the barrier layer, which, while offeringthe advantage of mechanical protecting the barrier layer, also entailsthe disadvantage that the outer surface of the plastic tube iscomparatively dull. This disadvantage is particularly aggravating if theplastic tube is shifted into empty tubes which are mostly formed ascorrugated tubes. Such “tube-within-a-tube” conduits are oftenprefabricated by the manufacturer, with plastic tubes of an overalllength of several tens of meters being inserted into corrugated tubes.Due to the dull outer surface of the plastic tubes, this insertionrequires relatively large forces. The process involves the risk that thedrive elements of the advance means for the insertion of the plastictube into the corrugated tube will rub against the plastic tube,possibly heating the plastic tube and thus damaging the protective layerthereof. Further, during the automatic prefabrication process at themanufacturing site, a sliding of the drive elements of the advance meanson the plastic tube disadvantageously prevents that the exact presetlength of plastic tube is reliably inserted into the corrugated tube.

From DE-U-81 04 908, a plastic tube is known which comprises a base tubeof cross-linked PE, an oxygen diffusion barrier layer from EVOH and anouter layer of a linear PE. Also such a tube causes relatively largefrictional forces while inserted into a corrugated tube or a similarencasing tube.

From U.S. Pat. No. 4,299,256, it is known to admix silicone oil to thePVC outer layer of a plastic tube so as to reduce the friction on theouter surface of the tube.

To sum up, with regard to PEX tubes applied on an adhesive layer and anEVOH layer thereon, with the layer thicknesses of these outer layersbeing each about 100 μm, the following difficulties arise:

Due to the varying bending stresses in the curve regions of the laidtubes, fissures may be generated as a result of the brittleness. Thiscan lead to a detachment of the EVOH layer and a propagation of thefissures and in the worst case may result in breakage of the tube.

The oxygen barrier property of the EVOH layer material is considerablydependent on the humidity in the environment of the tube. Notably, withincreased humidity absorption of the EVOH material, its barrier propertywill decrease correspondingly.

When connecting a tube to a connecting means, improper handling cancause damage to edges of the connecting means.

Further, certain connecting techniques require the tube to be widened.This type of connection technology finds increasing acceptance on themarket since it is performed easily and at low production costs.However, when widening a plastic tube provided with an EVOH barrierlayer, a danger exists that the EVOH layer might tear because itsexpansion behavior cannot be reproduced with sufficient reliability.

It is an object of the invention to provide a multi-layer plastic tubehaving an oxygen diffusion barrier layer which is protected againstmechanical damage, with the plastic tube being suited for insertion intoan outer tube in a relatively simple manner and without excessiveapplication of force.

According to the instant invention, the above object is solved by amulti-layer plastic tube comprising

a base tube,

a barrier layer surrounding the base tube and being impermeable tooxygen, and

a protective layer applied on the barrier layer for protecting thebarrier layer from damage, wherein the material of the barrier layerincludes a lubricant.

According to the invention, the material of the protective layer has alubricant admixed thereto which, mixed with the rest of the material ofthe protective layer, is applied onto the barrier layer by extrusion.During the solidifying of the material of the protective layer and inthe solidified condition, the lubricant migrates to the surface of theprotective layer and becomes active there, i.e. it lends its slidingproperties to the multi-layer plastic tube. Thus, the lubricant isparticularly a material on the basis of oleic-acid amide/erucic-acidamide. Alternatively, use can be made of materials on the basis ofsilicone oil or tetrafluoroethylene.

Due to the addition of a lubricant to the material of the protectivelayer which is preferably a material on PE-basis, the multi-layerplastic tube of the invention—in addition to the mechanical protectivefunction of the barrier layer—is given sliding properties whichfacilitate the insertion of the multi-layer plastic tube into an emptytube. The combination of a plastic material and a lubricant material inthe manufacture of plastic items is known e.g. from slidable films inthe packaging industry. Surprisingly, it has been found that thelubricants added in the packaging industry can be used, substantially inthe same concentrations, also for protective layers of plastic tubes.This could not have been expected as a matter of course because themanufacturing processes of films and plastic tubes are noticeablydifferent from each other, and especially because plastic tubes have asubstantially larger surface relative to the material volume used thanis the case with plastic tubes.

The term “lubricant” in the context of the invention is meant to includealso a lubricant in the form of a so-called antiblock agent which, bychanging the surface structure, allows two items to rub against eachother in a smooth manner. Such antiblock agents are known e.g. from theproduction and processing of plastic films.

Preferably, the base tube is arranged in an outer tube which is spacedfrom the outer protective layer of the base tube. The outer tube is madeof plastic and particularly is formed as a corrugated tube.

By way of alternative, further according to the invention, a variant ofthe above described embodiment of the multi-layer plastic tube comprises

a base tube,

a barrier layer surrounding the base tube and being impermeable tooxygen, and

a protective layer applied on the barrier layer for protecting thebarrier layer from damage, and

an outer tube having the base tube arranged therein and surrounding theprotective layer of the base tube with a clearance,

wherein the inner side of the outer tube is provided with a lubricant.

According to this variant of the invention, the lubricant is arranged onthe inner side of an outer tube which is suitably provided as acorrugated tube. In this case, the lubricant can be included in thematerial of the outer tube. If the lubricant is a material on the basisof oleic-acid amide/erucic-acid amide, the material migrates to theinner side of the outer tube where it will fulfill its sliding functionwhen the base tube with its dull outer protective layer is inserted intothe outer tube.

In the two above mentioned variants, depending on the material selectedfor the barrier layer, it is of advantage if an adhesive layercomprising an adhesive is arranged between the barrier layer and thebase tube. Preferably, the material of the adhesive is a modified linearLDPE.

A further variant of the invention relates to a multi-layer plastic tubecomprising

a base tube of PE, particularly of cross-linked PE,

an adhesive layer comprising an adhesive and surrounding the base tube,

a barrier layer surrounding the adhesive layer and being impermeable tooxygen, and

a protective layer of EVOH applied on the barrier layer for protectingthe barrier layer from damage,

the adhesive comprising a modified linear LDPE material of a densitybelow 926 kg/m³,

the protective layer comprising a modified linear LDPE material of adensity larger than 928 kg/m³, and

the modifications of the two linear LDPE materials being selected suchthat the adhesion of the protective layer to the EVOH barrier layer islarger than the adhesion of the EVOH barrier layer to the adhesivelayer.

In this variant of the invention, the barrier layer is formed of EVOHwhile the adhesive and the protective layer respectively comprisedifferently modified linear materials. The modifications are selectedsuch that the adhesion of the protective layer to the EVOH barrier layeris larger than the adhesion to the adhesive layer and thus to the basetube. Particularly, the density of the linear LDPE material of theadhesive layer is below 926 kg/m³ and larger than 916 kg/m³. Preferably,the density of the modified linear LDPE material of the protective layeris larger than 928 kg/m³ and below 940 kg/m³.

As has been found out, the above selection of the material for theprotective layer is advantageous in that the protective layer on the onehand is strong enough to protect the underlying brittle EVOH barrierlayer from mechanical damage and on the other hand is hard enough tolend sufficient sliding properties to the tube. Thus, the protectivelayer has sliding properties, i.e. in this regard is wholly made of amaterial having such a property, i.e. the modified linear LDPE material.

Further, as an advantageous modification of all variants of theinvention, it is provided that the barrier layer is arranged outside theneutral zone of the multi-layer plastic tube, i.e. off the center of theextension of the wall thickness of the multi-layer plastic tube.

All of the components of the multi-layer plastic tube of the inventionare extruded, wherein the extruded base tube is first provided with thebarrier layer by extrusion, and the protective layer is then extrudedonto the barrier layer.

Preferably, the layer thickness of the adhesive is between 10 μm and 70μm. The thickness of the protective layer should not be below 40 μm andthus is preferably between 40 μm and 100 μm.

Using the above described sequence of layers comprising the adhesivelayer, the barrier layer and the protective layer around the base tube,it is accomplished that a possible fissure generated in the barrierlayer will more or less “peter out” and thus cannot anymore attack thebase tube arranged under the adhesive layer. The reason for thisphenomenon of the “petering out” of a fissure is the use of an adhesiveon the basis of a linear LDPE having the above mentioned density, withthe thickness of this adhesive layer being suitably between 10 and 70μm.

Surprisingly, the thickness of the protective layer influences theoverall properties of the multi-layer plastic tube. The protective layershould not be thinner than 40 μm.

Basically, the protective layer should be made of a relatively toughmaterial, thus allowing to make optimum use of its protective functionfor the barrier layer. Thus, a linear LDPE would actually seem useful asa material for the protective layer. However, the tube surrounded withsuch a protective layer would become relatively dull and thus could notbe inserted by the desired lengths e.g. into a corrugated tube or thelike outer tube. Further, a protective layer of a relatively toughmaterial would generate a relatively soft surface throughout the tube,which in turn would cause deformations particularly when winding up thetube. This would give rise to phenomena such as flattened regions on theouter surface. This would have a negative influence on the opticalquality of the tube, possibly accompanied by disadvantages during theassembly with respect to the requirement of a tight connection of e.g. aconnecting element externally on the tube.

According to the invention, the problem of a relatively high frictionalresistance can be solved by adding a lubricant, while the problem of arisk of deformation is acted against by the use of high-density plasticmaterials on the basis of metallocene. The overall combination of thecompositions of the materials of the three layers wound around the basetube, i.e. of the adhesive layer, the EVOH barrier layer and theprotective layer, offers the following properties:

good adhesion of the EVOH layer,

considerable toughness of the material forming the outer surface of thetube,

high resistance against the propagation of fissures,

high stiffness of the material forming the outer surface of the tube toavoid deformation,

good sliding properties to allow insertion of the tube by the respectivedesired lengths into outer tubes, particularly corrugated tubes.

Two preferred embodiments of the invention will be explained in greaterdetail hereunder with reference to the drawings, wherein

FIG. 1 is a longitudinal sectional view of a tube-within-tubecombination comprising a gas-or fluid-conducting inner tube and an outercorrugated tube, and

FIGS. 2 and 3 are systematic illustrations of the method steps forthe—e.g. automatic—insertion of the inner tube into the outer tube.

FIG. 1 shows a sectional view of a tube-within-tube combination 10comprising an inner tube 12 and a corrugated outer tube 14. The innertube 12 comprises a base tube 16 of PE having an adhesive layer 18,like-wise on PE basis, applied thereon. A relatively brittle EVOHbarrier layer 20 is applied to the outer surface of adhesive layer 18.For protecting the EVOH barrier layer 20, this layer is surrounded by aprotective layer 22 made from a relatively tough material and comprisinge.g. the material of adhesive layer 18. The material of protective layer22 has a lubricant of oleic-acid amide basis added thereto. Thus, inspite of the protective layer 22 which is relatively tough and dull incomparison with the EVOH barrier layer, the inner tube 12 is givenextraordinarily good sliding properties. Also the outer tube 14 providedas a corrugated tube comprises polyethylene.

Due to its good sliding properties, the plastic inner tube 12 of theabove configuration can be prefabricated as a tube-within-tubecombination 10 with considerable lengths. The frictional resistance ofthe outer surface 24 of inner tube 12 and the inner surface 26 ofcorrugated tube 14 is reduced so that, when shifting the two tubes intoeach other according to FIGS. 2 and 3, considerably smaller forces arerequired than in a case where the material of the protective layer 22has no lubricant admixed thereto.

Thus, using the above described inner tube 12, there is provided amulti-layer plastic tube which in spite of the provision of a mechanicalprotective layer for the oxygen-diffusion barrier layer 20 has goodsliding properties.

On the basis of the order of layers of the multi-layer inner tube 12 asdescribed with reference to FIG. 1, this tube was produced using thefollowing composition of materials. For the base tube 16, a crosslinkedPE material was used. The adhesive layer 18 was extruded onto base tube16. The material used for the adhesive layer 18 was a modified linearLDPE material. This material was applied in a layer thickness of 10 μmto 70 μm, particularly 50 μm. Subsequently, the EVOH barrier layer 20was extruded onto adhesive layer 18. The thickness of the EVOH barrierlayer 20 was 50 μto 100 μm. Then, a modified linear LDPE material wasextruded onto the EVOH barrier layer 20 as a protective layer 22,notably in a thickness of at least 40 μm and particularly 100 μm at themost.

The modification of the LDPE material of base tube 16 was selected suchthat the adhesion of base tube 16 to barrier layer 20 was set in acontrolled manner but not in an optimal manner. On the other hand, themodification of the LDPE material of protective layer 22 was selected tohave the protective layer 22 attached to the EVOH barrier layer 20 withlarge adhesion. The density of the LDPE material of protective layer 22was higher than the density of the LDPE material of base tube 16.Particularly, the density of the LDPE material of base tube 16 wasbetween 918 and 924 kg/m³ while the density of the LDPE material ofprotective layer 22 was between 929 and 936 kg/m³. In addition to thegood connection between the individual layers of inner tube 12, theselection of the material for protective layer 22 offered, on the onehand, a good mechanical protection of the brittle barrier layer 20 withsufficiently high resistance against surface deformation and, on theother hand, good sliding properties of inner tube 12 so that the lattercould be shifted into the outer plastic corrugated tube 14 without highfrictional forces disturbing this manufacturing process fortube-within-tube conduit systems.

What is claimed is:
 1. A multi-layer plastic tube comprising: a basetube, a barrier layer surrounding the base tube, said barrier layerbeing impermeable to oxygen, and including a lubricant, and a protectivelayer applied on the barrier layer for protecting the barrier layer fromdamage, the material of the protective layer being an adhesive having alubricant added thereto.
 2. The multi-layer plastic tube according toclaim 1, wherein the lubricant comprises oleic-acid amide, erucic-acidamide, silicone oil or tetrafluoroethylene.
 3. The multi-layer plastictube according to claim 1, wherein an adhesive layer is arranged betweenthe base tube and the barrier layer.
 4. The multi-layer plastic tubeaccording to claim 1, wherein an adhesive layer is arranged between thebase tube and the barrier layer.
 5. The multi-layer plastic tubeaccording to claim 1, wherein the barrier layer is arranged off a centerof the extension of a wall thickness between the inner side of the basetube and an outer side of the protective layer.
 6. The multi-layerplastic tube according to claim 1, wherein all of the base tube, thebarrier layer and the protective layer are extruded.
 7. The multi-layerplastic tube according to claim 1, wherein the base tube is arranged inan outer tube surrounding the protective layer of the base tube with aclearance.
 8. The multi-layer plastic tube according to claim 1, whereinthe protective layer comprises metallocene.
 9. A multi-layer plastictube including, a base tube of cross-linked PE, an adhesive layersurrounding the base tube, a barrier layer surrounding the adhesivelayer, said barrier layer being impermeable to oxygen, and a protectivelayer applied on the barrier layer for protecting the barrier layer fromdamage, the improvement comprising: the protective layer including amodified linear LDPE material of a density below 926 kg/m³, and theprotective layer including a modified linear LDPE material of a densitylarger than 928 kg/m³, the modifications of the two linear LDPEmaterials being selected such that the adhesion of the protective layerto the EVOH barrier layer is larger than the adhesion of the EVOHbarrier layer to the adhesive layer.
 10. The multi-layer plastic tubeaccording to claim 9, wherein the density of the linear LDPE material ofthe adhesive layer is between 916 and 927 kg/m³.
 11. The multi-layerplastic tube according to claim 9, wherein the density of the linearLDPE material of the protective layer is between 927 and 940 kg/m³. 12.The multi-layer plastic tube according to claim 9, wherein the thicknessof the adhesive layer is between 10 μm and 70 μm.
 13. The multi-layerplastic tube according to claim 9, wherein the thickness of theprotective layer is between 40 μm and 100 μm.